F I L L E R A N D E X T E N D E R U S E S

Filler and Coating Pigmentsfor Papermakers

Ian Wilson

INTRODUCTIONHistorians attribute the Egyptians in the Nile Delta with preparingthe first type of paper, from a marsh grass called Cyperus papyrus.They cut the plant’s stem into thin strips, softened them in the river,and then pounded them into thin sheets that were then left to dry inthe sun. The resulting sheets were ideal for writing on, and Egyp-tians, Greeks, and Romans used them for record keeping, spiritualtexts, and works of art. Papyrus is the basis of the word paper. Thefather of papermaking is T’sai Lun, who in 105 AD experimentedwith a wide variety of materials and refined the process of macerat-ing the fiber of plants until each filament was completely separate.He mixed the individual fibers with water in a large vat with ascreen that was submerged and then lifted up through the water,catching the fibers on its surface; when dried, this thin layer ofintertwined fiber became what we today call paper. The art ofpapermaking spread to the Middle East and to Egypt, where itreplaced papyrus in the 9th century, followed by Morocco and thenSpain (about 1150) and France (1190).

Industrial minerals play a major part in the manufacture ofmodern paper. Originally, because they were less expensive thanfiber, minerals such as calcium carbonate (chalk) and kaolin wereused as fillers to reduce production costs. Although cost is still animportant factor, minerals have become “functional fillers” thatimpart specific properties to paper, such as improved printability,brightness, opacity, and smoothness. In paper coating, minerals areused as white pigments to conceal the fiber, thereby improvingbrightness, whiteness, opacity, and smoothness.

PAPER INDUSTRY STRUCTUREWorld ProductionIn 2003, world production of paper and paperboard was 339 Mtcompared with 239 Mt in 1990. A split of this production for 2003(Table 1) indicates similar levels from North America, Europe, andAsia. Between 1990 and 2003, North America’s share of the worldmarket fell from 37% to 30%, whereas Asia increased from 25% to32% as both Western and Eastern Europe decreased from 33.7% to31%. The world average annual growth in the production of paperand paperboard during the period from 1990 to 2002 was +2.8%,with Asia showing the largest growth of +5.2%. Of the 339 Mt(Table 1), Asia is now the leading producer with 32%, just overtak-ing North America and Europe with 30% and 31%, respectively.The top ten producing countries in the world now account for 73%

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of production (Paperloop 2004), led by the United States (24%),China (12%), Japan (9%), Canada (6%), Germany (6%), Finland(4%), Sweden (3%), South Korea (3%), France (3%), Italy (3%),and all other countries (27%).

The growth of the Chinese market from 1992 to 2003 was dra-matic, especially when compared with Japan (Figure 1): the growthrate was 13.1% in China, and the rate decreased by 1.3% in Japan.The Chinese growth was mainly driven by major companies invest-ing in new paper mills in China (including Asia Pulp and PaperCompany [APP], UPM-Kymmene, Stora Enso, and Oji Paper) com-bined with the fast-growing, apparent consumption per capita—in2003 paper consumption was 36 kg in China, an increase of 24%over 2002. This is still well below the levels of the United Stateswith 301 kg, the United Kingdom with 207 kg, as well as manyother countries; and just ahead of Indonesia and India with 23 kgand 6 kg, respectively.

Fibrous Plant Materials Used in PapermakingAlthough almost any plant material can be used for papermaking,very few are used because a number of factors determine whatmakes a good raw material:

• The plant must be abundant, inexpensive, and, if a waste prod-uct, of little use to others. It must grow in an accessible placeand should grow quickly.

• It must contain a high proportion of cellulose fibers, and itsstructure must allow the fibers to be isolated from the rest ofthe plant material with reasonable ease and without undueexpenditure of chemicals or heat.

Table 1. Paper and paperboard production by region in 2003

Region Production, kt% of World Production

(rounded up)

Asia 110,585 32

Europe 104,093 31

North America 100,280 30

Latin America 16,254 5

Australasia 3,871 1

Africa 3,672 1

Total 338,755 100

Adapted from Paperloop 2004.

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• The fiber itself when isolated should be suitable for paper-making, which means that it should be long and strong, anddevelop strength on beating. At the same time, it should becapable of being bleached to a good color without undue lossof strength.Of the many species of plants in the world, the following meet

the above requirements and are commonly used in papermaking:• Seed hairs—cotton• Bast fibers—flax, hemp, jute, ramie• Wood fibers—coniferous and deciduous woods• Leaf fibers—esparto, manila, sisal• Grasses—bamboo, bagasse

Potential new pulp sources are being developed all the time.Malaysia, for example, announced in March 2003 that the world’sfirst oil-palm–based pulp plant would be set up in Sabah (Borneo)with the capacity to produce 25,000 t of pulp. A new pulpingmethod using empty fruit bunches (EFBs), which are currently awaste product from the palm oil industry, will be developed using

Adapted from Paperloop 2004. Figure 1. Growth in paper market for China versus Japan, 1993 to 2003

15,000

20,000

25,000

30,000

35,000

40,000

45,000

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

Ktpa

2002–2003 –1.3%

China

Japan

2002–2003 +13.1%

caustic soda technology. If successful, Malaysia has the potential toproduce 3 Mtpy of pulp from EFB alone.

Main Paper GradesTable 2 summarizes the main types of paper grade, their fiber com-position, pigment filler and coating loading, and their end uses.Coated paper is coated on one or both sides with a mix of clay andcarbonates to create a high quality printing surface. Coated papercan be fine, lightweight, medium weight, or machine finished.Uncoated fine paper is used principally for printing and writing.Woodfree, freesheet, or fine paper is paper used by the graphicindustry for writing, including office paper such as photocopyingand laser printing paper; these may be coated or uncoated.

Leading Producers of Paper and PaperboardThe paper industry continues to consolidate, with the top 10 com-panies accounting for an increasing proportion of world paper andboard capacity. Production by the 10 leading companies in 2003accounted for 27.9% of global output (Table 3).

In 2001, Finnish-owned Stora Enso increased its capacity to15.1 Mtpy and International Paper closed down 2.2 Mtpy of itsless-efficient capacity; in the same year, UPM-Kymmene acquiredHaindl, a leading European producer of publication paper. Publica-tion paper includes newsprint, coated, and uncoated papers(mainly lightweight coated [LWC] and SC). This further consoli-dation increased UPM-Kymmene’s capacity from 8.285 Mtpy to11.705 Mtpy by 2003. UPM-Kymmene is now the world’s largestmanufacturer of magazine paper (LWC and SC) with annualcapacity of 5.465 Mt, or about one-quarter of the global market formagazine paper.

MAJOR PIGMENTS USED IN PAPERMAKINGThe major pigments used in printing and writing paper (P&W;includes SC, MFC, WFU, and WFC) are calcium carbonate (pre-cipitated calcium carbonate [PCC] and ground calcium carbonate[GCC]), kaolin, and others (talc, TiO2, and others). Harris (2004)estimated that 30 Mt of these major pigments were used worldwide

Table 2. Paper grades, fiber composition, pigment and coating loading, and end uses

Paper Grade Fiber Raw Material Pigments End Uses

Newsprint De-inked pulp and/or mechanical pulp Filler loading up to 12%, originally from de-inked pulp

Newspapers, inserts, flyers (advertising)

Specialty newsprintBooks, papersHigh brightness

De-inked pulp and/or mechanical pulp Filler loading <10%; specialty pigments can be used as well

Newspaper supplements, newspapers, books, directories, advertising

Supercalendered (SC) papersSC B, A, and A+ grades

Mechanical and chemical pulp Filler loading up to 35%

Filler loading up to 10%; coating 25%–30% of paper weight

Multicolor magazines, catalogs, supplements, inserts, advertising materials; used in gravure and offset printing

Coated mechanical papers (also called machine-finished coated [MFC] paper)

Mechanical and chemical pulp Filler loading up to 10%; coating typically from 20%–35% of paper weight

Magazines, catalogs, supplements, books, advertising materials

Woodfree uncoated (WFU) papers

Chemical pulp Filler loading up to 25% Office papers, writing papers, envelopes, direct mail, magazines, books, advanced materials

Woodfree coated (WFC) paper; also coated fine paper (art printing paper)

Chemical pulp, possible to use some chemi-thermomechanical pulping (CTMP)

Filler loading up to 15% and double/triple coating

Magazines, brochures, direct mail, annual reports, books, advertising materials; higher quality books, reports

Specialty papers Chemical pulp Filler load and coating dependent on grade

Label papers, label release papers, food wrapping, packaging

Kraft papers Chemical pulp No pigments Sacks, bags, wrapping and packing, envelopes

Source: Haarla 2002.

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in 2002 (Harris 2004), and Figure 2 shows the breakdown of use.Of the 30 Mt of pigment used, 18 Mt was for coating pigments andthe split was GCC (53%), kaolin (35%), PCC (9%), and others(3%; this was mainly talc, titanium dioxide, and a few others). Theremaining 12 Mt is used as a filler.

ROLE OF PIGMENT PROPERTIES IN PAPERMAKINGFor the papermaker, the critical pigment properties are

• Physical properties (also includes optical properties such asbrightness, yellowness, and shade; coverage; ink absorption;and others)— Particle size and shape— Particle-size distribution— Aspect ratio (platiness or blockiness—kaolin can be platy

and blocky but calcium carbonate is generally rhombohe-dral [hexagonal] for marble [crystalline limestone])

• Pigment moisture (for pigment handling)• Pigment hardness—abrasiveness (wear on wire, doctor and

slitter wearing)• Residues, impurities, contamination—origin can be from pro-

cessing the industrial mineral, from transportation, and fromother sources (“runnability”* in paper machine or coating orcalendering causes streaks and breaks, which are expensive)

ROLE OF FILLERS IN PAPERFillers are highly desirable in printing papers because they increasethe opacity, raise the brightness, and generally improve printingproperties. The main types of mineral filler for acid papers are talc,hydrous kaolin, calcined kaolin, precipitated silicas and silicates(PSS), and titanium dioxide. For neutral/alkaline papers, talc,hydrous kaolin, calcined kaolin, PSS, titanium dioxide, GCC, andPCC are used. The use of fillers is important when opacity isneeded at a low-basis weight, and they are invaluable in packaginggrades where low permeability is combined with opacity to protectfood from light. There are many fillers: barite, GCC (based onchalk, limestone, and marble), PCC, kaolin, pyrophyllite, mica,gypsum, plastic pigment, satin white, alumina, and titanium diox-ide. The properties of the filler derive from the ability of the fillerparticles to refract and backscatter light through the surface of thesheet. If the filler is not evenly dispersed through the sheet and floc-culates in small clumps, then the optical efficiency of the filler willbe reduced. If the z-direction distribution of the filler is uneven,then the sheet may appear two-sided. The whiteness of the fillerrelates to the dominant wavelength of the light. Excluding someuncoated book-publishing grades, papers tend to have a blue white-ness. This requires adding a blue or violet dye to shift the shade ofthe paper into the desired region of the spectrum.

Because dyes reduce brightness, high-brightness filler must beused or optical brightening agents added for premium grade, suchas company stationery and direct mail papers. The importance ofavoiding filler flocculation emphasizes the point that fillers are notsimply inert optical entities but interact with other additives, notonly in terms of their own distribution but also to influence sheetstructure such as formation, bulk, pore structure, and surface topog-raphy (texture). Aside from their optical effects, fillers or fillerblends can be used to improve aspects of product uniformity and

* Newspapers and magazines require long runs on the printing presses, andseveral factors affect how well a paper will run on the press. Runnabilitydescribes a paper’s ability to hold ink on its surface consistently and toabsorb ink uniformly, along with its dimensional stability and its surfacetexture.

quality. An understanding of filler interactions with retentions aids,sizing agents, cationic starch, and the dynamics of the wet andforming systems is required (Jopson and Moore 2004).

The main driving force for filler in fine papers is to substitutemore expensive fiber with filler. No filler is capable of producingmaximum light scattering for brightness and opacity without hav-ing any detrimental impact on wet web strength and sheet physicalproperties. The best pigments for brightness and opacity debondfibers the most because of their inherent high surface area.

Kaolin, calcium carbonate (GCC and PCC), and talc are themost widely used mineral fillers, with regional variations depend-ing on local resources available. Table 4 lists the pigments used asfiller in different printing paper applications. In the United States,PCC is widely used as filler because of the wide availability oflimestone; the lime produced from limestone is converted into PCCat a satellite plant adjacent to a paper mill. Filler pigments musthave a high degree of whiteness, a high index of refraction, smallparticle size, low solubility in water, and low specific gravity. It isalso important that the filler be chemically inert to avoid reactionswith other components in the sheet and in the papermaking system.The filler should contain a minimum of impurities, and the grit con-tent must be low to avoid excessive wear of the wire and other pro-cessing equipment such as cutting blades. Furthermore, unless thefiller has very unusual properties, it must be inexpensive.

Hydrous KaolinKaolin was the usual filler used in Europe and the United States upuntil the 1990s when the use of PCC in the United States and GCC

Table 3. World’s 10 leading producers of paper and paperboard in 2003

Position Company Output, ktpy

1 Stora Enso 13,960

2 International Paper 13,844

3 UPM-Kymmene 10,232

4 Svenska Cellulosa (SCA) 9,725

5 Georgia Pacific 8,843

6 Weyerhaeuser 8,558

7 Oji Paper 7,900

8 Nippon Unipac Holding 7,835

9 Smurfit Stone Container Corporation 7,307

10 Abitibi Consolidated 6,421

Total 94,625

Adapted from Paperloop 2004.

Source: Harris 2004.Figure 2. World pigment use in P&W paper (2002)

Talc, TiO2,and Others

7%2.1 Mt

Calcium Carbonate (CC)55%

16.5 Mt

Kaolin38%

11.4 Mt

GCC38%

(70% of CC)11.5 Mt

PCC17%

(30% of CC)5 Mt

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Table 4. Filler pigments in different printing paper applications

Type of Paper Ash Content, % Clay, % GCC, % PCC, % Talc, %

Newsprint* <15 <10 <10 Can be used

Machine-finished specialty (MFS) <15 <10 <10

Supercalendered offset (SCO) <35 <35 <10 <20

Supercalendered rotogravure (SCR) <35 <35 <10 <20 <10

Lightweight coated offset (LWCO)† <10 <10 <10 <10 <10

High-brightness lightweight coated (HB LWC)† <12 <10 <10 <10 <10

Medium-weight coated (MWC)† <10 <10 <10 <10

WFU† <22 <22

WFC† <15 <15 <15 Pitch talc

Adapted from Haarla 2002.* Most of the ash comes from recycled fiber in newsprint, if the main raw material is recycled paper.† A part of the ash comes from coated broke in coated paper.

in Europe emerged. The main use of kaolin fillers in the last decadehas been in engineered products. The new range of filler content forSCA paper is based on the production of a high-aspect-ratio (often>40) filler and a controlled particle-size distribution of about 50 wt %<2 µm and a minimum of 5% >10 µm. These high-aspect-ratiokaolin products, with a brightness of ISO 82, provide good opacityto the base paper. They are particularly useful in SCA papers wherethe platy kaolin with calendering gives a sheet that can competewith LWC papers. The development of a range of platy kaolins hasbeen at the expense of the less expensive kaolin filler.

Over the past two decades, additional processes have beendeveloped to improve the quality of the kaolin from the deposits inCornwall in the United Kingdom. These deposits have been welldocumented in the literature (Exley 1959, 1976; Sheppard 1977;Halliday 1980; Allman-Ward et al. 1982; Alderton and Rankin1983; Bray and Spooner 1983; Allman-Ward et al. 1985; Bristowand Exley 1994; Bristow 1995; Manning, Hill, and Howe 1996;Scott, Hart, and Smith 1996; Psyrillos, Manning, and Burley 1998;Psyrillos et al. 1999; Bristow et al. 2000; Thurlow 2001; Bowditch,undated). For brightness enhancement, superconducting magnetshave been introduced alongside high-intensity magnetic separators(HIMSs), froth flotation, selective flocculation, and selective separa-tion processes to remove abrasive materials such as quartz and feld-spar. The most significant processing development over the lastdecade, however, has been the production of delaminated clays fromvermiform or stacky kaolinites so that approximately 60% of theprocessed Cornwall kaolin is now delaminated. The flow process atlow-abrasion plants involves flotation to remove contaminants (feld-spar, quartz, and mica) and a sand grinder (known as such because itonce used a round resistant sand as the grinding medium; a ceramicbead is now the preferred medium) for delamination.

The aspect ratio of the resultant delaminated kaolin is an impor-tant parameter, and Imerys developed a stop-flow conductivity mea-surement instrument that gives a shape factor (called a factor becauseaspect ratio is not actually measured). The method is known as PAN-ACEA (particle assessment [by] natural alignment [and] conductivityeffect analysis). Measurements of shape factor are made online andhelp to control the process. Historically, aspect ratio was measuredusing the transmission electron microscopy (TEM) platinum shadow-ing technique, which was a very lengthy process relying on the parti-cle thickness being proportional to the shadow length—the thinnerthe platelet of kaolin, the narrower the shadow.

The base paper pigment has an important bearing on the sub-sequent coating pigment application. The influence of the basepaper structure is more noticeable when the paper is coated with

GCC because its particles pack less efficiently on the surface com-pared to the platy and broad particle distribution kaolin (Bown1991; Lorusso 2002; Hiorns and Nesbitt 2003). Although it is clearthat coating color formulation and coating conditions have poten-tially greater influence on the coated paper quality, the filler contentof the base paper has a profound effect on coating runnability andpaper quality.

Calcined KaolinCalcined clay is used in small amounts in newsprint and as an addi-tion to other fillers to replace more expensive titanium dioxide toimprove opacity. With the onset of calcined clay as filler in news-print, it is common now to have newspapers with color photographsnot visible through the sheet because of its increased opacity.

Calcium CarbonateThe conversion from acid to alkaline papermaking techniques andthe demand for brighter and bulkier paper have been the main driv-ers behind the increased preference for calcium carbonate overkaolin. This switch has eroded the share of the market held by kaolinbecause paper producers partly substitute its use with calcium car-bonate, which is less expensive and often brighter. Neutrally sizedpaper can have higher mineral filler loadings than acid-sized paper,so calcium carbonate slurries are preferred over kaolin slurriesbecause of their higher solids content. Table 5 compares the proper-ties of kaolin and calcium carbonate, both precipitated and ground.Although calcium carbonate is generally brighter than most com-mercial kaolin, new grades of kaolin for use in paper coating have abrightness of more than 90%.

Ground Calcium Carbonate

Table 6 lists some of the advantages of GCC compared to kaolinusage in alkaline woodfree papermaking. Kaolin was once the mostwidely used filler in paper manufacture, but the last two decadeshave seen a steady increase in the use of calcium carbonate. Theconversion from acid to alkaline papermaking and the demand forbrighter paper have been the main reasons for this change.

Precipitated Calcium Carbonate

World PCC capacity is approximately 6.2 Mtpy, of which almostthree-quarters is used in paper. Nearly all of the PCC in paper isused as a filler, and the largest market is the United States. Aspapermakers transferred to alkaline technology, the number of sat-ellite PCC plants has increased significantly since the first U.S.plant opened in 1986. By 2000, some 80 plants had been installed

Filler and Coating Pigments for Papermakers 1291

Table 5. Comparison among properties of kaolin, PCC, and GCC in papermaking

Property Kaolin GCC PCC

Brightness 80%–85% (some 90%) >90%–96% 90%–97%

Particle size Naturally 2 µm Requires grinding Manufactured fine

Opacity Excellent Moderate at high load High at high load

Loading levels 20%–30% 20%–30% Limited to 20%

Sheet strength Good Excellent Moderate

Bulking Moderate Good Good

Absorption Low Low High

Chemical reactivity Inert Unstable in acid environments Unstable in acid environments

Flexibility Filler/coating Alkaline-only filler/coating Mainly filler

Processing Extensive Grinding/sizing Energy intensive

Availability Restricted Geologically plentiful Satellite plants

Price Low (North America) Low (Europe) Based on cost-effectiveness

Source: Harben 1998.

worldwide, with almost 50 in North America alone; the industry isdominated by MTI with 54 plants, followed by Huber EngineeredMaterials with 12 and Imerys with 6.

PCC manufacturing in Europe is shared between five majorproducers with an estimated total production capacity of more than2.0 Mtpy in 2003, including products for paper and other appli-cations. The 5 companies are Huber Engineered Materials with6 plants, Specialty Minerals Inc. (SMI) with 13 plants, Omya with4 plants, Solvay with 6 plants, Schaefer Kalk with 3 plants, andImerys with 1 plant. At the time of this printing, Huber is in the pro-cess of selling their 6 PCC plants. The competitiveness of an on-sitePCC plant is primarily influenced by the size of the plant (economicsof scale) and by the CO2 content in the gas source from the hostpaper mill. This means that an on-site satellite PCC plant has to havea certain minimum size and be supplied with a gas of a certain mini-mum CO2 content in order to be economically viable. For example,an on-site PCC plant has to process a minimum of 20,000 tpy to beeconomically justified, corresponding to the demand from paper pro-duction of 100,000 tpy of uncoated paper with a 20% filler level.

In Europe, PCC has shown by far the strongest growth rateand since 1995 has continuously taken market share from GCC andother fillers. The growth of PCC is likely to continue with furtherpenetration as filler into the WFU paper segment, although at aslower rate as the market matures.

Some paper mills that had been using GCC derived fromchalk in Europe were among the first to capitalize on the addedbrightness that could be achieved using PCC. Several PCC satelliteplants have come onstream in Asia since the mid-1990s, includingplants in Thailand, Indonesia, Japan, China, South Korea, andMalaysia. New satellite plants have also been built in South Africaand in South America.

PCC is now making some inroads as filler in groundwood (SCand LWC) papers, and this represents the largest remaining poten-tial market. This market is currently dominated by kaolin and talc,however, especially in European rotogravure paper. MTI hasinvested much research into developing acid-tolerant PCC, allow-ing its entry into the groundwood paper sector, for which it now hasseveral satellite plants.

TalcThe choice of filler in paper is driven by cost reduction and qualityimprovement. For talc, which is more expensive than some otherpigments, improving the paper quality and the papermaking pro-cess itself is the dominate driving factor. The properties of talc—

soft, organophilic, chemically inert, and platy—are reasons why ittoday is used as filler in many different kinds of paper. The organo-philic surface helps reduce dye consumption and two-sidedness incolored paper. Two-sidedness in colored paper is the term used todescribe the difference in color characteristics between the top sideand the bottom side of the sheet. The difference can be either inshade or in strength, or both. Contributing factors are finish, chemi-cal additives, points of addition, order of addition, and colorantcharacteristics. With careful control of all these factors, however,two-sidedness can be eliminated to result in a uniform sheet.

The relatively coarser particle-size distribution of talc (com-pared with other pigments) leads to better retention in the sheet andlower effect on the paper’s strength properties. Improved dewatering,less wire abrasion, higher retention, a longer life of cutting knives,and fewer core breaks in SCR printing are typical advantages in theprocess provided by talc. In addition, the pitch and sticky controlfunction reduces tacky deposits and improves paper machine run-nability. The paper quality itself is influenced by talc with increasedsmoothness, better printability, deeper color in colored papers, and alower impact on strength properties than other fillers.

Other Filler PigmentsOther mineral systems are employed—for example, titanium diox-ide. Titanium dioxide is used more as a specialty chemical to impart

Table 6. Advantages of GCC compared to kaolin in alkaline woodfree papermaking

Paper

• Brightness—GCC has a higher brightness than clay; lower optical brightening agent (OBA) demand without alum

• Strength—tends to be higher without high amounts of alum; filler loadings higher

• Permanence—absence of alum benefits aging properties of paper

Process

• Refining—30% energy savings in refining under slightly alkaline conditions

• Drainage—rhombohedral shape of GCC drains better than platy kaolin

• Drying—dries better than clay because of slightly more hydrophobic nature of GCC

• Water—better drainage and lower bacteriological activity reduce water demand

• pH stability—strong buffering action; GCC keeps pH level stable at 7.2 to 8.4

1292 Industrial Minerals and Rocks

specific end-use properties rather than as filler because of its cost. Itis used primarily for its opacity properties, particularly in light-weight papers such as paper used for bibles.

A new specialty filler in newsprint is Zeocros PF, produced byIneos Silicas, a leading global supplier of silica, silicate, and zeoliteproducts. Zeocros PF has a high ISO brightness of 97; small, angularparticles with efficient light-scattering surfaces; and a tight particle-size distribution. In newsprint it raises opacity, improves printability,reduces print through/strike through, and enhances brightness. A 1%filler addition will give 0.6 points of opacity increase. Paper withZeocros also prevents ink penetration into the paper.

Gypsum, or calcium sulfate, is an abundant mineral formed nat-urally and from industrial by-products (such as flue gas desulfuredgypsum and phosphogypsum). It generally has the disadvantage ofhigh solubility, and it also tends to plug machine felts. Calpak C20 isfiller derived from gypsum in Spain and is being developed byKemira. It is a bright filler for a wide range of pH and sizing systems.The production of filler gypsum, however, is limited in the world.

Diatomaceous earth is not filler in the sense of the other pig-ments but is normally used for pitch control (0.5%–1.5% on pulp),to improve formation, and to increase the rate of drainage, thoughone serious disadvantage is its abrasiveness.

THE ROLE OF PIGMENTS IN PAPER COATINGApplication of Coating PigmentsA coating layer on a paper surface can bring the following improve-ments to the paper sheet:

• Cover the base paper fibers to give a uniform surface

• Make the paper whiter

• Contribute to the paper’s opacity

• Give the desired finish—gloss, silk, or matte

• Give the desired printing properties

A coating mixture, known as a coating color, is normally a mix-ture of a mineral with a binder to stick the mixture to the paper, a vis-cosity modifier to assist in the application to the paper surface, andoften other chemicals that improve the shade of the coating and itsprinting properties. An important characteristic for any coating coloris good rheological response during metering and application and anability to retain water during application. An ideal coating mixtureneeds to be fluid during application, yet immobilize quickly aftermetering. An ideal coating should cover all the paper fibers. Fibercoverage is often achieved by using high coatweights, coarse parti-cles, platy particles, and coating mixtures that immobilize rapidly.

In paper coating, minerals are used as white pigments to con-ceal the fiber, thereby improving brightness, whiteness, and opac-ity, as well as smoothness. If applied by a blade coater, thepigmented layers impart a fairly constant surface layer, but of vary-ing depth as the surface voids and pits of the base sheet are filled in.There is a growing understanding that the filler in the base papercan have a great influence on the behavior of the coating pigment.Application methods such as the air knife and, to a degree, the filmpress or metering size press provide a coating of more uniformthickness, which follows the larger-scale surface contours of thesheet to give a varying surface level. Such contour coating methodsare particularly useful in applying a ground coat to uneven sub-strates such as folding carton board and corrugating liners.

Coating using the size press is a growing practice, producingpaper that bridges the gap between uncoated and full-blown blade-coated products. An extension of this is using high surface area sil-ica or calcium carbonate pigments to provide dye-receptive coat-ings for inkjet paper (pigment acts as an acceptor of the image

medium). Even in conventional offset lithographic grades, carefuldevelopment has been undertaken over many years to optimize theink transfer and drying characteristics of the coatings. In electro-photographic printing grades, controlling surface resistivity andtoner adhesion is critical in coatings development. In the Indigounit with its wet toner system, surface chemistry, specifically acid-base interaction, is critical in image transfer to the paper surface.Matching the pigment formulation to the imaging systems is anincreasingly important part of product development, especially inhigher value coated paper grades.

The coating formulation must also be optimized for runnabil-ity on a high-speed coating line. This requires careful attention torheology (viscosity at high and low shear). Such parameters areimportant not only to coating transfer and metering but also to uni-formity of the coating in terms of coverage of the fiber in the basesheet. This is vital to print quality. The rheology of the coating isdetermined by the interactions between the pigment particles them-selves and between pigment and binder under the influence of coat-ings solids content and temperature. Common binders includestyrene-butadiene rubber (SBR) lattices, starch, acrylics, or vinylpolymers. The rheology can be manipulated by water-phase thick-eners such as carboxyl methyl cellulose (CMC) and associativethickeners such as alkali-swellable acrylates or hydrophobicallymodified urethanes.

In most paper coating, the pigment concentration is higherthan the critical pigment binder concentration (CPVC) that is famil-iar in paint formulations. A blade coating for offset lithographicprinting would contain 100 parts pigment to 14 parts binder at60%–65% solids. A coating for air knife or rod coater applicationwould be about 30%–40% solids. The objective is to create amicroporous coating than can facilitate ink transfer. For folding car-ton board, acrylic and polyvinyl acetate binders are used to increaseporosity to permit the use of adhesives on the coated surface.Exceptions to the higher pigment binder ratio are found in barriercoating. Here pigments are used not for their optical properties butto increase the tortuosity of the diffusion path of oils, grease, orwater through the barrier. Pigments with high aspect ratios such aplaty clay and talc find applications in this area. The addition levelis generally about 30–40 parts pigment to 100 parts emulsion poly-mer, well below the CPVC. A balance has to be struck betweenadding pigment to boost barrier properties and preserving coatingelasticity, so that the paper and board can be folded without crack-ing the barrier layer.

Table 7 lists pigments used in the coating process and theirbasic properties. Kaolin and GCC are the major coating pigments,accounting for some 90% of the total. Basically, GCC and kaolinare blended in a wide range of differing coating formulations,depending on the type of paper being manufactured.

KaolinKaolin has a platy morphology that is still required for a large num-ber of paper applications, particularly in lightweight coated papers.The trend in recent years has been to combine different minerals inone coating formulation. Kaolin can be mixed with GCC, withPCC, and more recently, with talc, to obtain improved performance.If a choice is to be made between kaolin and GCC for coating, thepapermaker considers the solids percentage (the higher the solids,the less drying of paper necessary), paper brightness, paper opacity,fiber coverage, paper gloss, and print gloss. For high-brightnesspaper, GCC is used; but for fiber coverage, paper gloss, and printgloss, the platy nature of kaolin is preferable. Kaolin is widely usedin paint as an extender, and the calcined grades give higher opacitythan a hydrous type. There are regional trends, with the United

Filler and Coating Pigments for Papermakers 1293

Table 7. Properties of pigments used in coating paper

Pigment CompositionRefractive

IndexSpecificGravity

Dry Brightness,%

Average Particle Size D50, µm Crystal Form

Kaolin Al2O3•2SiO2•2H2O 1.55 2.65 70–91 at least 70% <2 Pseudo-hexagonal

Calcined clay Al2O3•2SiO2 1.62 2.70 90 ~2, aggregate Chunky aggregate

Natural GCC CaCO3 1.49–1.66 2.72 90–96 0.8–1.5 Rhombohedral

PCC CaCO3 >95 0.1–0.2 Scalenohedral (C)

Calcite (C) 1.49–1.66 2.72 Rhombic (C)

Aragonite (A) 1.53–1.68 2.94 Acicular (A)

Talc 3MgO•4SiO2•H2O 1.57 2.75 85–90 ~50% <2 Monoclinic, platy

Gypsum CaSO4•2H2O 1.52 2.34 85–90 at least 70% <2 Monoclinic, prism

Titanium dioxide TiO2, rutile (R) 2.70 4.20 97–98 0.2–0.5 Tetragonal

TiO2, anatase (A) 2.55 3.90 98–99 0.2–0.5 Tetragonal

Alumina Al(OH)4 1.57 2.42 98–99 0.3–1.0 Monoclinic, platy

Satin white Calcium sulfo-alumina complex 1.46 1.55 >90 ~90% <2 Acicular

Blanc fixe BaSO4 1.69 4.3–4.5 98 0.2–2.0 Orthorhombic

Zinc sulfide ZnS 2.37 3.98 97–98 0.3–0.5 Generally hexagonal

Zinc oxide ZnO 2.01 5.65 97–98 0.3–0.5 Hexagonal

Plastic pigment Polystyrene 1.59 1.05 >97 0.1–0.5 Spherical

Adapted from Dean 1997.

States still relying dominantly on kaolin for coating, followed byPCC and GCC. In Europe and Asia, the trend has been towardGCC, no doubt because of the proximity of high-quality marbledeposits in such places as Carrara in Italy and Ipoh in Malaysia.

The world kaolin market of high-quality beneficiated kaolinwas estimated at 25 Mt for 2003 (Wilson 2003, 2004b).The majorproducing kaolin companies worldwide (Table 8) are led by Imeryswith 25% of the market. Leading kaolin-producing countries arethe United States, mainly based on the sedimentary deposits inGeorgia, with 36%; United Kingdom, 10%; Brazil, 9%; and othercountries, 45%. Brazil has shown the most significant growth and isexpected to soon overtake the United Kingdom. Large reserves ofhigh-quality coating kaolin discovered in the Amazon Basin havebeen developed over the past 20 years. These deposits are all sedi-mentary in origin and are widespread throughout parts of the Ama-zon Basin. The main operations in the Amazon Basin are CADAM(now owned by Companhia Vale do Rio Doce [CVRD], which isthe largest exporter of iron in the world); Para Pigmentos SA(PPSA; now 100% owned by CVRD); and Rio Capim Caulim(RCC; 100% owned by Imerys). In 2005, these three companieshad an installed capacity of 2.25 Mtpy, split between CADAM(0.8 Mt), PPSA (0.6 Mt), and RCC (0.85 Mt). Sales in 2001brought in revenue of US$200 million. Proven reserves of kaolinare put at >500 Mt, with CADAM having 270 Mt of ultrafine clay(98 wt % <2 µm); PPSA, 110 Mt of platy clay at 82–85 wt % <2 µm(excluding other reserves that CVRD controls in the same region);and RCC, with 120 Mt of platy-type kaolin at 78–94 wt % <2 µm.Future expansions based on these large high-quality reserves areplanned with CADAM aiming to produce 1 Mtpy by 2007; withPPSA, 1 Mtpy; and RCC, 1 Mtpy by mid-2005. CVRD is emergingas the second largest kaolin company in the world, followingImerys.

The world coating clay market is estimated at 8 Mt, of whichthe major suppliers are the United States, Brazil, and the UnitedKingdom, with some production from Australia, China, the CzechRepublic, Bulgaria, Germany, and France. Coating clays based onsedimentary kaolin sequences in the United States (Georgia), and inthe Amazon Basin show a wide range of properties but generally

Table 8. Summary of some No. 1 and No. 2 properties based on brightness and particle size distribution of coating clays from the United States, Brazil, and Australia

ProductTAPPI Brightness, GE %

(unless stated otherwise)Particle Size,

% <2 µm

U.S. Clays

No. 1 high bright 90–92 90–94

No. 1 fine high bright 90–92 96–100

No. 1 86.5–88 90–94

No. 1 fine 87–89 95–99

Range of No. 1 products 86.5–92 90–100

No. 2 high bright 90–92 80–84

No. 2 85.5–87 80–84

Range of No. 2 products 85.5–92 80–84

Coarse delaminated 85–87 54–62

Delaminated 88–90 75–81

Delaminated high glossing 88–90 83 minimum

Range of delaminated clays 85–90 54–83

Brazilian (Amazon)

CADAM—Premier clay 89 (ISO) 98

PPSA—Century 90–91 80

RCC (Capim)—Imerys

Engineered pigments (fine)

Capim DG 90.5 90

Capim GP 90.5 90

Coating high-brightness (delaminated)

Capim NP 90.5 80

Capim CC 89.0 80

Australia (Pittong, Victoria)

EckaPlate (HB) S—delaminated 86 (ISO) 85

EckaPlate HB—delaminated 85 (ISO) 85

EckaCote—delaminated 84 (ISO) 85

1294 Industrial Minerals and Rocks

Table 9. Range of Imerys coating clays from the United Kingdom, United States, and Brazil

United Kingdom, ISO brightness % United States, GE brightness % Brazil, GE brightness %

Engineered pigments Coating regular brightness Engineered pigments

Suprastar—88.5 Fine #1 Astra Glaze—88.0 Capim DG—90.5

Supraprint—88.5 #1 Premier—88.0 Capim GP—90.5

Coating high brightness

Suprawhite—95 88.0 Delaminated Coating high-brightness delaminated

Suprawhite—80 87.5 Astra-Plate—86.0 Capim NP—90.5

Coating regular brightness Capim CC—89.0

SPS—85.5

Ultra platy coating

Supraplate—86.5

Suprasmooth—65 83.0

Table 10. Range of Thiele Kaolin Company’s calcined, coating, and filler clays from the United States

Grade DescriptionTAPPI Brightness,

GE %Sedigraph,% <2 µm Viscosity*

Maximum Residue, % <325 mesh pH†

% Moisture Solids, dry 1.5% maximum

Calcined Clays

Kaoclay Calcined high-brightness coating and filler

92.0 minimum92.5 minimum

86–92 Not available 0.010 7.0–8.0 50–52

Kaoclay 80 Calcined low-brightness coating and filler

80–83 86–92 Not available 0.010 6.5–7.54.0–6.0

50–52

Coating Clays

Kaogloss 90 No. 1 high bright 90–92 90–94 300 0.010 6.0–8.0 69–71

Kaobrite 90 No. 2 high bright 90–92 90–94 300 0.010 6.0–8.0 69–71

Kaofine 90 No. 1 fine high brightness 90–92 96–100 200‡ 0.010 6.5–8.0 69–71

Kaogloss No. 1 86.5–88 90–94 300 0.010 6.5–7.5 69–71

Kaobrite No. 2 86.5–87.0 80–84 300 0.010 6.5–7.5 69–71

Kaofine No. 1 fine 87–89 95–99 300 0.010 6.5–8.0 69–71

Kaowhite Delaminated 88–90 75–81 450§ 0.010 6.5–7.5 67–68

Kaowhite S Delaminated high glossing 88–90 83 minimum 450§ 0.010 6.5–7.5 67–68

Kaowhite C Coarse delaminated 85–87 54–62 350** 0.020 6.5–7.5 62–64

Kaoprint Top coat high bright 90.5–92.5 89–94 500†† 0.010 6.0–8.0 68–69

Lopaque M Base coat low bright 76–82 88–96 300 0.010 6.5–8.0 69–71

Filler Clays

Kaofill HB Delaminated filler clay 87–89.5 77–83 Not available 0.010 6.5–7.5 68–69.5

Kaofill Coarse filler clay 83.0 minimum 50 minimum Not available 0.15 6.5–7.5 69–71

EG-44 Fine filler clay 81.0 minimum 88 minimum Not available 0.30 6.5–8.0 69–71

* Brookfield viscosity. No. 1 spindle, 20 rpm, typically @ 70% solids.† Slurry as shipped, dry @ 20% solids.‡ Number 2 spindle.§ Number 3 spindle @ 67.5% solids.

** Number 3 spindle @ 63% solids.†† Number 2 spindle. As shipped.

are clays with a high brightness and good runnability. There arealso ranges of platier clays that are either found naturally or can bedelaminated from stacky kaolinite found in the deposits.

A range of clays from the United States, Brazil, and Australiais shown in Table 8 and indicates the range of products, whetherthey have been delaminated or not, and the particle-size distribution.

Imerys, as the largest producer of kaolin with 25% of theworld capacity of 25 Mtpy, is the only international kaolin companyto produce coating clays from the United States, the United King-dom, and Brazil; Table 9 shows their range of products.

Table 10 shows a full range of kaolin products covering cal-cined clays, coating clays, and filler clays for the U.S. producerThiele Kaolin Company.

New kaolin deposits are being identified in many parts of theworld, including Ukraine, Suriname, China, and Australia.Although the coating kaolin market will be dominated by Brazilianand U.S. clays, there is the opportunity for potential new venturesto enter the market. Australia is seen as a likely source of high-quality coating kaolin, with deposits in Western Australia beingdeveloped. W.A. Kaolin Holdings Pty (WAK) has acquired the

Filler and Coating Pigments for Papermakers 1295

deposits evaluated in great detail by CRA/Rio Tinto in the Wick-epin Area, 180 km southeast of Perth. The firm has drilled 621boreholes in the area, amounting to almost 20,000 m of core. All ofthis core has been evaluated and models of the deposit prepared.Proved reserves of 100 Mt have been identified with the potentialfor an additional 300 Mt from 274 km2. Full-scale pilot-plant trialswere planned for late 2005, leading to a decision on whether a plantwill be constructed. The deposit is kaolinized granite with a highkaolin yield of 50% at the <45-µm refining level; by comparison,kaolinized granite from Cornwall shows a 15% yield, reflecting thedifferent levels of feldspar in the basement granites–gneisses ofWestern Australia and the high-level granites of southwestEngland. Detailed characterization studies have been carried out inU.K., Japanese, and U.S. laboratories, and the potential for high-brightness coating clay has been evaluated in helicoating trials inFinland and elsewhere. This processing has involved cyclone andcentrifuge separation with the coarse booklets of kaolinite from theunderflows (Figure 3) being subjected to delamination to give aplaty product (Figure 4).

Figure 3. Scanning electron microscopy (SEM) of the WAK Australian kaolin dynocone underflow showing kaolinite stacks

Figure 4. SEM of WAK-delaminated dynocone overflow platy kaolin

For the Chinese market, the high-brightness clays may beblended with GCC in a mix of 70% GCC and 30% kaolin. The West-ern Australian kaolin can be processed to give a high-brightnessdelaminated (HBD) coating clay and also as a high-brightness coat-ing clay (Figure 5). Table 11 compares properties of the WAK withBrazilian and U.S. sources, showing the high-brightness values of theWAK clay.

Some Chinese kaolin is suitable for coating clay but, atpresent, it supplies only the coated board market, not coated paper.A deposit supplying the coated board market is Maoming inGuangdong Province (Zhang et al. 1982; Yuan and Murray 1993;Wilson, Halls, and Spiro 1997; Wilson 2004a).

Kaolin that has been carefully processed to give a controlledparticle-size distribution is known as engineered kaolin. Ultrafine(less than 0.1 µm) particles are removed to improve light scattering(brightness). These particles are too fine to affect light scatter.Removing such particles does not affect sheet gloss because parti-cles from 1 µm to 0.1 µm have the greatest influence on this prop-erty. The disadvantage with engineered kaolin is that the spaces leftby the fine particles that have been removed must be filled withwater, leading to lower weight-percent solids and less water reten-tion. Because of its higher light scattering, engineered pigmentswill give improvements in coated sheet brightness and opacity(Table 12).

The shape of the kaolinite particle also influences the coatedsheet properties. Pigment shape refers to aspect ratio: the ratio ofparticle height to width. For a pigment with the same average parti-cle size as measured by a sedigraph, a platy or high-aspect-ratiopigment will impart more fiber coverage when coated. Thisimproved coverage often will result in a smoother coated sheet,which is extremely important for holding dot quality in rotogravureprinting. The disadvantage of platy pigments is that the large plate-like particles do not easily flow among one another, giving rise tolower weight-percent solids. The same plate-like characteristicsmean, however, that under a coating blade, platy pigments do notlose their water easily.

Ground Calcium CarbonateRoskill Information Services (2002) estimated that the demand forGCC in papermaking was 15 Mtpy in 2002, with Europe dominat-ing at 67%, followed by Asia and Oceania (22%), North America(8%), and others (3% for Middle East, Africa, and South America).

Figure 5. Processing of the WAK kaolin with brightness of various products

Cyclones HC Overflow

DynoconeUnderflowSG @ 35

and 70 Kwh/t

HC Underflow

DynoconeCentrifuge

DynoconeOverflow

Sand ground @35 & 70 Kwh/t

Magnet, bleach Magnet, bleach

Disperse Mixed Standard and HighBrightness Matrices

<53 µm feed

>500 µm, >250 µm, <500 µm,+53 µm, <250 µm

Magnet, bleachCalcination

@ 850˚C and1,150˚C

CalcinedClay Products

DelaminatedProduct A

DelaminatedProduct B

DelaminatedProduct C

CoatingProduct D

Dynoo/f gr

91.8 94.0 89.1 (ISO Brightness) 89.6 90.0 90.4

1296 Industrial Minerals and Rocks

Table 11. Comparison of Australian clays with Brazilian and U.S. coating products

HBD Coating Clays for Blending with GCC(HBD clays—fine particle size distribution, platy, glossy)

Australia Brazil United States

Product name HBD SBD Capim SP Hydragloss 90

Deposit Wickepin Wickepin Capim Georgia

Company WAK WAK Imerys Huber

Wt % <2 µm 94 94 91 98

Wt % <1 µm 76 81 70 98

Wt % <0.5 µm 41 56 42 92

Wt % <0.25 µm 13 28 19 61

ISO brightness 90.5 88.4 89.5 88.8

ISO yellowness 3.3 3.5 4.0 4.2

Shape factor 27 25 25 22

Non-Delaminated HB Coating Clays (HB clays—blocky)

Australia Brazil

Product name HB Century Amazon 90

Deposit Wickepin Capim Jari

Company WAK PPSA CADAM

Wt % <2 µm 90 82 98

Wt % <1 µm 75 61 96

Wt % <0.5 µm 49 33 83

Wt % <0.25 µm 15 10 46

ISO brightness 89.5 89 88

ISO yellowness 3.5 4.0 5.0

Shape factor 12 11 15

Table 12. Effect on paper brightness and opacity of engineered kaolin and GCC

Coating Pigment (magazine paper)

PigmentBrightness

PaperBrightness

PaperOpacity

Kaolin 88 70.8 85.2

Engineered kaolin 88 72.8 85.8

Ground marble (GCC) 95 71.4 84.0

Engineered GCC 95 73.4 85.5

Adapted from Gentile 2003.

Table 13. Coating pigment choice between kaolin and GCC based on properties*

Color Solids

Paper Brightness

Paper Opacity

Fiber Coverage

Paper Gloss

PrintGloss

Coarse GCC ++ + – + – – – –

Fine GCC ++ + – – – – –

Engineered GCC

++ + + – –

Blocky kaolin + – + ++ +

Coarse platy kaolin

– – + ++ +

Fine platy kaolin

– + + + ++

Engineered kaolin

+ + + + +

* Plus signs indicate most advantageous property in coating; minus signs indicate least advantageous property in coating. Blank cells indicate no particular advantage or disadvantage.

It can be seen that Europe is a major user of GCC basedmainly on marble deposits and also some chalk. Most of the coatingmarket is supplied by products derived from marble deposits, withOmya controlling 75% of the total market and Imerys, Provencale,Reverte, and others accounting for the rest. A major growth marketfor GCC is now Asia, of which China is at the forefront with manynew projects involving satellite GCC plants (for example, APP hasa 500,000-tpy GCC plant in Dagang).

Using GCC as a coating pigment gives technical and eco-nomic benefits such as high brightness, coating at higher solids,lower binder demand, good runnability, and improved printability.In some applications, the advantage of high-brightness coating pig-ments leads to lower hiding power and reduced opacity. Therefore,adding opacifying pigments to the coating color to reach therequired opacity is common practice. Pigments with high bright-ness, broad particle-size distribution, and average particle size ofmany of the GCCs currently used in coating are not optimal foropacity.

The larger companies such as Omya and Imerys are now pro-ducing a range of products for which the GCC is manufactured tooptimize the opacity. The new products are produced by a slightlydifferent processing technique: the particle-size distribution and theaverage particle size are moved according to the theory of lightscattering toward that of the ideal opacifying pigment. The process-ing involves techniques to give steeper curves and in some casesremoving finer particles. The percentage of solids at which the pro-cessing of the GCC is carried is also important.

Omya has developed a new pigment called Hydrocarb CC.Using the theory of light scattering, Omya designed a pigment witha tailor-made (engineered) mean particle size and particle-size dis-tribution. For high opacity, the target is a narrow particle-size distri-bution with a mean diameter between 0.6 and 0.8 µm. Using amodified grinding technology, Hydrocarb CC has been manufac-tured with a narrower particle distribution than Hydrocarb 90.Comparing Hydrocarb 90 and the new pigment shows that the nar-row particle size leads to a higher wet void volume and a lower spe-cific surface for Hydrocarb CC. This difference is explained by thereduced amount of fine particles, which results in a more open andporous surface for an increase in ink receptivity. Hydrocarb CCproduced from marble shows a higher brightness and opacity on thefinished paper compared to Hydrocarb 90 produced from the samesource of calcium carbonate.

The choice of mineral pigment used by the papermaker willdepend on a number of factors related to the paper grade being pro-duced. These include fiber production costs, mineral pigment price,paper optical properties, and strength. It is significant that thepapers considered the best in relative quality terms have the highestmineral content, whether it is by filler content or coating or both.Minerals add value to paper.

Table 13 shows a choice between kaolin and GCC, based onthe paper properties. Here GCC is clearly better for color solids andpaper brightness but has poorer paper gloss and print gloss thankaolin. The platy nature of the kaolin gives better fiber coverage(especially the coarse platy type for SCA papers), and the fine platykaolins give good paper gloss and print gloss compared to GCC.

Today the pigments used in paper coating are many and variedin their properties. Blending has become a common feature withprecoats often being a coarser GCC (about 60–70 wt % <2 µm) fol-lowed by a topcoat with a fine GCC (90–95 wt % <2 µm) blendedwith either a blocky or platy kaolin dependent on the type of sur-face required. Very fine clays, such as Amazon Premium, are oftenblended with 20–50 wt % fine GCC to give a higher brightnesssurface with enhanced printing properties. Various blends from

Filler and Coating Pigments for Papermakers 1297

Table 14. Typical blends of kaolin and GCC used in LWC and MWC paper

Paper Type

Blend of Kaolin and GCC

CommentsType of Kaolin Type of GCC

LWC 50% U.S. #1 (blocky) 50% 90 wt % <2 µm Single coated, 60-gsm paper

80% Platy clay 20% 90 wt % <2 µm Single coated

MWC/woodfree—gloss

Gloss, precoat None 100% 60–75 wt % <2 µm Base paper, 90 gsm

Gloss, topcoat 30% U.S. #1 blocky 70% 90–95 wt % <2 µm

MWC/woodfree—matte

Matte, precoat None 100% 60–75 wt % <2 µm Base paper, 90 gsm

Matte, topcoat 50% Fine platy kaolin 50% 90 wt % <2 µm

Table 15. Soft calcined clay products for the paper industry

Product Properties Brightness, % Size, wt % <2 µm Product Form

Engelhard

Luminex Ultra-high brightness calcined 95.8–96.5 80–90 Dry

Ansilex 93 High brightness calcined 92.5–93.5 86–90 Dry

Ansilex Standard brightness calcined 90.0–92.5 86–90 Dry, 51% solids

Excaliber Standard brightness, high opacity 80.0–82.0 86–92 Dry

Huber

Hycal Calcined 92.0–94.0 86–96 Dry

Hubertex Calcined 92.0–93.5 NR* Dry, slurry

Hubertex D Calcined, high solids slurry 92.0–93.0 NR* Slurry

Imerys

Alphatex Calcined 92.5 93 50% solids, dry

Alphatex HP Calcined 92 91 50% solids, dry

Opacitex Calcined 80 88 Dry

Deltatex Calcined 92.5 92 59% solids, dry

Liner-fil 300 Calcined 92.5 91 50% solids, dry

Astra-Plex Calcined composite NR NR 56% solids

Thiele

Kaocal High brightness calcined 92–83 86–92 50%–52%

Kaocal 80 Standard brightness 80–83 86–92 50%–52%

* NR = no results.

pigments such as GCC, PCC, kaolin, and talc are continuouslybeing developed to give the papermaker a wide choice. Table 14shows some typical blends of kaolin and GCC used for varioustypes of paper.

Unlike the situation in other areas where large players such asImerys, Engelhard, Huber, Thiele (United States); CADAM andPPSA (Brazil; for kaolin); Omya and Imerys (for GCC); and Spe-cialty Minerals dominate, the Chinese paper pigment market has alarger number of producers for all pigments, with no single com-pany having a sizeable market share. Development of PCC andGCC in China will be dependent on local sources of limestone andmarble, respectively, and also the logistics of delivering marble forsatellite GCC plants.

The largest paper mill in China is the APP operation atDagang. Here the pulp is imported from Indonesia and the basepaper is manufactured using PCC produced from lime from locallimestone in a satellite plant at the paper mill. The precoat is a GCCat approximately 65 wt % <2 µm, followed by a topcoat that is95 wt % <2 µm GCC, mixed with 30% imported kaolin from the

United States and Brazil. Calcined clay might well be used at the10% level in the precoat.

Calcined ClaySoft calcined clays are loosely aggregated as a result of fluxing thatoccurs on the edges of the particles. This aggregation results in par-ticles of nominally larger sizes but with entrained air voids that giverise to good light-scattering characteristics. These properties arewidely used in the paper industry to provide good opacity, inkimmobilization, and light-scattering effects. Table 15 shows themain calcined products sold into the paper industry, which is thelargest consumer of soft calcined clay products. These are all softcalcined products that are not reactive like metakaolins but havebeen agglomerated by heating. They have point-contact adhesion,which gives the kaolin platelets a three-dimensional structure; it isthis structure that is of benefit to the papermakers: they enhance theoptical and printing properties of paper. These products are used inrotogravure applications where low levels of calcined clay improveoptics and structure the coating to provide good fiber coverage and

1298 Industrial Minerals and Rocks

missing dot performance. Calcined clays are also used in newsprintapplications to improve the opacity and reduce the consumption oftitanium dioxide.

TalcIn Europe, the two main printing processes are rotogravure andoffset. The offset process poses problems for the use of talcbecause the transfer of the image uses a hydrophilic/hydrophobicaction, which is largely incompatible with an organophilic min-eral like talc. The rotogravure process, however, prints by directcontact between the image cylinder and the paper. Producingprinting cylinders is expensive, and, therefore, rotogravure print-ing is suitable only for large-volume magazines, mail order cata-logs, or flyers. Key requirements for rotogravure papers are goodspool formation and smoothness. Good spool formation is essen-tial if the printing presses are to run without constant stops. In theearly 1970s, papermakers exclusively used platy English and Ger-man clays, and then added U.S. delaminated clays. In the late1970s, the research department at Finminerals (now Mondo Min-erals) began working with large Finnish customers to develop talcgrades suitable for coating; in 1981 they launched their product.This was followed by Luzenac introducing a rotogravure-coatingtalc. The talc works in two ways to improve rotogravure paper.First, its low coefficient of friction allows the large spools to beproduced with a constant tension, which reduces paper breaks;and second, because talc is very platy, it helps improve thesmoothness and therefore gives better printability. Talc has suc-cessfully established itself as a major coating mineral for rotogra-vure paper (Whiteley 2002).

Precipitated Calcium CarbonatePCC is well accepted as a filler pigment and is now beginning tobe used as a coating pigment. Currently, PCC accounts for lessthan 5% of coating pigments used but the market is developing, ledby SMI, Imerys, and Omya. SMI, part of Minerals Technologies,pioneered the concept of the satellite plant for PCC and now hasmore than 50 plants in operation around the world. PCC is valuedfor its high brightness and light-scattering characteristic in paperfilling and coating applications. PCC is produced through a reac-tion process that uses very pure calcium carbonate crystals andwater. The crystals can be produced in a variety of different shapesand sizes, depending on the specific reaction process that is used.The two main raw materials required are quicklime and carbondioxide. Because quicklime is readily available in many areas ofthe world, it can be delivered to the satellite plant at a reasonablecost. In most paper mills, carbon dioxide is available to be usedfrom a mill flue gas. This makes PCC economically attractive forthe paper mill.

For paper coating, SMI developed several PCC productlines—ALBAGLOS, OPACARB, and JETCOAT. ALBAGLOS hasa precisely controlled particle-size distribution, good high-shearrheology, and good ink hold-out, and it can be used at high levels toimprove paper brightness and opacity while maintaining sheetgloss. OPACARB has an acicular shape that provides good smooth-ness, gloss, brightness, and opacity owing to a narrow particle-sizedistribution and a mean particle size of 0.40 µm. JETCOAT is thelatest PCC coating pigment and is designed specifically for coatinginkjet and other nonimpact papers. SMI has developed six differentcommercial crystal shapes of PCC that give a wide variety of crys-tal structures that offer different performance characteristics(brightness, opacity, and bulk) to a sheet of paper. These shapes arescalenohedral calcite, spherical calcite, prismatic calcite, clusteredacicular aragonite, rhombohedral calcite, and discrete acicular ara-

gonite. Using crystal-engineered PCC can also improve papermachine productivity.

Imerys produces a broad range of PCC products for filling,precoating, and top/single coating, applicable in all paper- andboard-manufacturing processes. Imerys’s PCC coating productsinclude Opti-Cal Print (rhombohedral), with a mean particle size(mps) of 0.5–0.7 µm and a brightness of 95–98 GE. Opti-Cal Matteis rhombohedral with an mps of 2.0–2.2 µm and Opti-Cal Gloss isaragonite with an mps of 0.3–0.5 µm and a brightness of 95–98 GE.Imerys was chosen by M-real, one of Europe’s foremost producersof P&W papers (ranked first in WFC paper, and second in WFUpaper), as the supplier of calcium carbonates for its paper mill inHusum, Sweden, which has a capacity of 620 ktpy. Under a long-term contract, Imerys completed construction in 2005 of a satellitePCC plant at M-real’s Husum facility.

Blanc FixeBlanc fixe (meaning stable white) is a synthetic barium sulfate thatis precipitated with a defined particle size from highly purifiedsolutions of barium salts and sodium sulfate. Because of its chemi-cal production process, blanc fixe is free of impurities such asquartz. The feed material for the process is chemical-grade barite(BaSO4), which is generally low in silica. Much of the chemical-grade barites used in Europe for blanc fixe production come fromChina. With its hardness of 3 on the Mohs scale, blanc fixe has lowabrasion and exhibits an extremely high light reflectance in broadranges of the spectrum, not only in the visible but also well into theultraviolet and infrared ranges. Blanc fixe is generally 99% BaSO4with an average particle size of 3 µm, a lightness/color of 98.5 (onthe International Commission on Illumination [CIE] L*a*b* colorsystem), and a pH of 9. It is used both in pulp and as a pigment incoating because of its color and low binder requirements. It is usedin small amounts in coated art papers and in photographic papers,where it was applied to the base to improve the surface before thelight-sensitive photographic emulsion is applied. The developmentof extrusion coating involving special plastics that do not absorb theprocessing chemicals, however, has generally replaced this precoat,especially for color printing paper.

Satin WhiteSatin white, a calcium sulfo-aluminate, is one of the oldest knownpigments for paper. Manufacture involves reacting alum withslaked lime at a controlled temperature. Satin white forms acicularcrystals that are often 1–2 µm long and 0.1–0.2 µm in cross section.This shape imparts an open, bulky structure to the coating that has amajor influence on its optical properties; in particular, it is responsi-ble for high-gloss development during calendering as well as givinga high print gloss, high ink receptivity, high bulk, and good cover-ing power. A 4-g/m2 coating of satin white would have a similarcovering power to a 10-g/m2 clay coating. Satin white, however,has a very high dispersant demand because of calcium ions in solu-tion. It also leads to high adhesive demand if the satin white levelexceeds 10%. The main applications for satin white are in high-quality art paper and high-quality lightweight offset grades.

Titanium DioxideTiO2 offers the highest opacity and hiding power of all coating pig-ments, giving good optical density, excellent brightness, and very lowgrit levels (0.0010–0.0050 wt % of <325 mesh). High brightness,combined with superior light scattering, provides excellent bright-ness and whiteness to paper and paperboard products. TiO2 providesthe best opacifying performance available to the paper industrybecause of highly efficient light scattering. The higher the TiO2 g/m2

Filler and Coating Pigments for Papermakers 1299

light scattering, the less TiO2 needed to achieve the opacity target.The high refractive index of the rutile TiO2 crystal proves an inherentlight scattering advantage over anatase TiO2. The median particlesize of TiO2 coating pigments is very fine at just 0.30–0.60 µm.

THE FUTUREPigments for paper will continue to be a growth industry over thenext decade, particularly in China, where production is expected togrow from 33 Mtpy to 80 Mtpy by 2015 (M. Sang, personal com-munication). In Western Europe, paper consumption increased sig-nificantly from 1950 to 2000, despite innovations such astelevision, computers, and the Internet that were expected to lead toa paperless society (Figure 6). Unless there is a dramatic shortageof fiber in the world for some unknown reason, papermaking, par-ticularly the uncoated woodfree papers, is expected to grow.

Kaolin competes with GCC, PCC, and talc in the paper indus-try. In 1980, kaolin accounted for 87% of the pigment used in paper(10.2 Mt), whereas in 2000 this had fallen to 40% when the totalpigment used in paper was 29.9 Mt. During the same period, GCC’smarket share grew from 9% to 33% and PCC use grew from virtu-ally zero to a 14% market share (Haarla 2002). This trend towardcalcium carbonate has mainly been a result of an alkaline paper-making system replacing an acid system, and also because of therequirements of higher brightness pigments for woodfree pulp.

These changes have been in a market that has grown signifi-cantly, however, so the tonnages that the kaolin companies supplyhave remained much the same over the last decade. The supply ofkaolin has switched from the United States and the United King-dom to Brazil with the discovery and development of world-classfacilities based on the sedimentary kaolin deposits in the AmazonBasin.

There will be no shortage of pigments in the world for thenext 100 years at least. New kaolin deposits in the Amazon Basin,Western Australia, and elsewhere are being discovered, and poten-tial resources are very large. The Cornish kaolin deposits mined formore than 250 years have just another 40–50 years of reserves left.There is sufficient suitable limestone available to manufacture limefor PCC production, and the use of GCC, which relies mainly ongood sources of marble, is expected to grow. Although there mightwell be some shortages of high-quality marble in some parts of theworld, increased processing such as flotation to remove silicates,graphite, and iron pyrites will allow the use of lower quality marble

Adapted from Haarla 2002.Figure 6. Western European newsprint and writing paper consumption, Mtpy

Newsprint

P&W Papers

ColorTV

Satellite TVMini-Computers

Cable TVPCs

Laser PrintersCD ROMs

Expansion of the Internet

Wireless Application Protocol (WAP) Mobile Phones

TV, RadioCinema

MainframeComputers

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

10

20

30

40

deposits. New deposits of anatase and rutile are being discoveredand developed, which means no shortage of titanium dioxide.

The real value of minerals used in papermaking lies in theircapability to further improve printing results and enable the paperor board substrate to be a good printing surface. Developing higherbrightness, whiteness, and opacity remains a challenge, especiallyas papers and boards trend to lower basis weights and sheet thick-ness. The digital revolution has enabled almost everyone to accesstext and pictures that can then be used to produce an infinite varietyof printed media. This capability has created new requirements forpaper and board surface properties than can be satisfied not only bysurface-applied pigments but also by the use of fillers to improvebase sheet uniformity. Developing filled barrier coatings is also anemerging opportunity.

REFERENCESAlderton, D.H.M., and A.H. Rankin. 1983. The character and

evolution of hydrothermal fluids associated with the kaolinisedSt. Austell granite, SW England. Journal of the GeologicalSociety of London 140:297–310.

Allman-Ward, P. 1985. Distribution of uranium and thorium in thewestern lobe of the St. Austell granite and the effects ofalteration processes. In High Heat Production (HHP): Granites,Hydrothermal Circulation and Ore Genesis. London: Institutionof Mining and Metallurgy.

Allman-Ward, P., C. Halls, A.H. Rankin, and C.M. Bristow. 1982.An intensive hydrothermal breccia body at Wheal Remfry in thewestern part of the St. Austell granite pluton, Cornwall. Pages1–28 in Metallisation Associated with Acid Magmatism. Editedby A.M. Evans. Chichester, UK: Wiley.

Bowditch, I. Undated. A compendium of facts and figures relatingto the modern china clay industry and its history. Imeryspamphlet (unpublished).

Bown, R. 1991. Physical and chemical aspects of the use of fillersin paper. Pages 162–197 in Paper Chemistry. Edited by J.C.Roberts. Glasgow and London: Blackie.

Bray, C.J., and E.T.C. Spooner. 1983. Sheeted vein Sn-Wmineralisation and greisenisation associated with economickaolinisation, Goonbarrow china clay pit, St. Austell, Cornwall,England: Geological relationships and geochronology.Economic Geology 78:1064–1089.

Bristow, C.M. 1995. The genesis of the china clays of south-westEngland—a multistage story. Pages 171–203 in Kaolin Genesisand Utilization. Edited by H.H. Murray, W.M. Bundy, and C.C.Harvey. Boulder, CO: Clay Minerals Society.

Bristow, C.M., and C.S. Exley. 1994. Historical and geologicalaspects of the china clay industry of southeast England.Transactions of the Royal Geological Society of Cornwall21:247–314.

Bristow, C.M., Q. Palmer, G.J. Witte, I. Bowditch, and J.H. Howe.2000. The ball clay and china clay industries of SouthwestEngland in 2000. Pages 17–41 in Industrial Minerals andExtractive Industry Geology. Edited by P.W. Scott and C.M.Bristow. London: Geological Society.

Dean, T.W.R, editor. 1997. The Essential Guide to AqueousCoating of Paper and Board. Cirencester, UK: BuckwayPrinting.

Exley, C.S. 1959. Magmatic differentiation and alteration in the St.Austell granite. Quarterly Journal of the Geological Society ofLondon 114:197–230.

———. 1976. Observations on the formation of kaolinite in the St.Austell granite. Clay Minerals 11:51–63.

1300 Industrial Minerals and Rocks

Gentile, E. 2003. Clays as fillers and coatings for paper. Presentedat Euro Clays Workshop, European Clay Minerals GroupMeeting, Modena, Italy, June.

Haarla, E. 2002. Trends in papermaking: Pigment demand outstripspaper. Industrial Minerals 414:44–53.

Halliday, A.N. 1980. The timing of early and main stage oremineralisation in southwest Cornwall. Economic Geology59:752–759.

Harben, P.W. 1998. CaCO3 in paper, PCC versus the competition.Industrial Minerals 366:39–49.

Harris, R. 2004. Minerals in paper—looking east for growth.Industrial Minerals 443:52–57.

Hiorns, A., and T. Nesbitt. 2003. Particle packing of blocky andplaty pigments—a comparison of computer simulations andexperimental results. Presented at the 8th TAPPI AdvancedCoating Fundamental Symposium, Chicago, May.

Jopson, N., and G. Moore. 2004. The cutting edge: Papermakingwith minerals. Industrial Minerals 485:38–41.

Lorusso, M. 2002. Improving SC paper with engineered pigment.Pulp and Paper 76(11):45–48.

Manning, D.A.C., P. Hill, and J.H. Howe. 1996. Primarylithological variation in the kaolinised St. Austell granite,Cornwall, England. Journal of the Geological Society of London153:827–838.

Paperloop. 2004. Annual Review of Paper and PaperboardProduction. Bedford, MA: Paperloop.

Psyrillos, A., J.H. Howe, D.A.C. Manning, and S.D. Burley. 1999.Geological controls on kaolin particle shape and consequencesfor mineral processing. Clay Minerals 34:193–208.

Psyrillos, A., D.A.C. Manning, and S.D. Burley. 1998.Geochemical constraints on kaolinisation in the St. Austellgranite, Cornwall, England. Journal of the Geological Society ofLondon 155:829–840.

Roskill Information Services. 2002. The Economics of GroundCalcium Carbonate. London: Roskill Information Services.

Scott, P.W., F.W. Hart, and D. Smith. 1996. The quantitativemineralogy of ceramic grade kaolin from the St. Austell graniteand its relationship to chemistry and physical properties.Proceedings of the Ussher Society 9:091–096.

Sheppard, S.M.F. 1977. The Cornubian Batholith, south westEngland: D/H and 18O/16O studies of kaolinite and otheralteration minerals. Journal of the Geological Society of London133:573–591.

Thurlow, C. 2001. China Clay from Cornwall and Devon—theModern China Clay Industry. 3rd edition. St. Austell, Cornwall,UK: Cornish Hillside Publications.

Whiteley, G. 2002. Talc’s renaissance in European paperproduction. Pages 71–74 in Proceedings of the 15th IndustrialMinerals International Congress. Surrey, UK: IndustrialMinerals Information.

Wilson, I.R. 2003. Kaolin. Mining Annual Review 2002. London:The Mining Journal Ltd.

———. 2004a. Kaolin and halloysite deposits of China. ClayMinerals 39:1–15.

———. 2004b. Kaolin. Mining Annual Review 2003. London: TheMining Journal Ltd.

Wilson, I.R., C. Halls, and B. Spiro. 1997. A comparison betweenthe china clay deposits of China and Cornwall. Proceedings ofthe Ussher Society 9:195–200.

Yuan, J., and H.H. Murray. 1993. Mineralogical and physicalproperties of the Maoming kaolin from Guangdong Province,South China. Pages 249–259 in Kaolin Genesis and Utilization.Edited by H.H. Murray, W.M. Bundy, and C.C. Harvey.Boulder, CO: Clay Minerals Society.

Zhang, Z., D. Lu., M. Feng, B. Feng, and T. Jin. 1982. Kaolindeposits of China. Pages 719–731 in Proceedings of the 1981International Clay Conference. Sedimentology 35. Amsterdam:Elsevier.